1. Technical Field
The present invention relates to lithography, and particularly to a reticle for use in a lithography system.
2. Description of Related Art
Three main types of exposure techniques used for transferring patterns from the reticle to the photoresist include: contact type, proximity type and projection type. For the semiconductor technique of deep sub-micron, the precision of the exposure technique is highly limited to photo-resolution. This limitation may cause error or even failure to the pattern transferring. Taking proximity exposure technique as an example, the original pattern of the reticle lines are often different from the transferred pattern of the lines on the photoresist after the exposure step.
If the lines of the reticle are isolated, i.e., the gap spaces between the adjacent isolated lines are large such that the space/line ratio is higher than about 6.5, skilled persons locate assist features at the outside of every isolated line to gain sufficient process window thereof. A sufficient process window ensures that the pattern of the transferred lines are substantially the same as the pattern of the reticle lines. If the reticle lines of the reticle are dense, i.e., the gap spaces between the adjacent dense lines are small such that the space/line ratio is lower than about 3.0, skilled persons locate assist features at the outside edges of the extreme outer lines, but locate no assist feature between the dense lines. This is because locating an assist feature between dense lines easily induces sidelobe problems or increases mask-error factors.
However, in the semi-dense region of the reticle, i.e., the region where the space/line ratio is equal to about 3.0-6.5, skilled persons encounter a catch-22 situation. That is, inserting no assist feature between the semi-dense lines gains insufficient process window, however, on the other hand, inserting an assist feature easily induces sidelobe problems or increase mask-error factors.